Field
The technology of the disclosure is related to transparent conductive oxide (TCO) films and particularly to aluminum doped zinc oxide (AZO) TCO films.
Technical Background
Transparent conductive oxide (TCO) films are widely used in display and photovoltaic applications where electrical conductivity in layers transparent to visible and near infrared are required. For example, TCO films are used on the cover glass of LCD flat panel displays for addressing the liquid crystal cells, as sensing electrodes in resistive and capacitive touchscreens, and as the transparent contact in photovoltaics. TCO's are also widely used for infrared (IR) mirrors in low emissivity (low-E) glass windows,
Indium-tin oxide (ITO) has been the predominant TCO for high-end applications due to its low resistivity (about 1 to 2×10−4 Ohm·cm), low surface roughness (less than 5 nm Ra), environmental durability and ease of patterning. However, the scarcity of indium makes the ITO film expensive, limiting its application. Achieving practical industrial application of lower cost alternatives based on more abundant elements has been a major focus of research in the past several decades.
APCVD-deposited (atmospheric pressure chemical vapor deposition-deposited) fluorine-doped tin oxide (FTO) is widely used for low-E windows and for some photovoltaic (PV) applications where the roughness and defect rates of the APCVD process—performed in-line on the flat glass produced by the “float process”—can be tolerated. Research on off-line processes (those not performed in-line on the float process or other glass production process) has focused mainly on doped zinc oxide materials. LPCVD (low-pressure CVD) has been used to deposit boron-doped zinc oxide TCO to produce high roughness TCO films optimized for the front contact layer in silicon tandem PV. Sputtering is in use or under investigation for producing aluminum-doped zinc oxide (AZO) and gallium-doped zinc oxide (GZO) films for industrial application. These sputtered zinc oxide processes may find a role in CdTe PV or touch applications where smooth films are more desirable, in contrast to the higher roughness films optimal for silicon tandem PV. Etching the relatively smooth doped zinc oxide sputtered films has also been demonstrated to produce controlled roughness suitable for silicon tandem PV (Berginski et. al. J. Appl. Phys. 101, 074903 (2007)).
The properties of AZO have been extensively reviewed by Elmer (Elmer, J. Phys. D: Appl. Phys. 34 (2001) 3097). The lowest reported resistivity is on the order of 2.0×10−4 Ohm·cm, and the highest Hall mobility of 60 cm2V−1s−1. The optical properties and electrical properties of AZO are not independent—they are linked physically and can be described by the Drude model (see Singh, J. Appl. Phys. 95(7) (2004) 3640). This model explains the thermal as well as electrical and optical properties of metals by the movement of both free and bound electrons. An increase in the free electron concentration leads to higher conductivity. However, it also increases absorption in the infrared (in lower frequency radiation) as the plasma frequency (the frequency above which materials become transparent to EM radiation in the Drude model) increases.
The sputter deposition of AZO has been reported from metal and from ceramic targets, using RF, MF, pulse DC, DC, and RF superimposed DC sputtering. Deposition from metal targets requires precise control of the reactive process to achieve a consistent level of non-stoichiometric oxygen content. AC processes using a twin magnetron and optical emission control are used for large in-line AZO coating. DC sputtering processes with metal targets are more challenging to control because target poisoning produces hysteresis. DC sputtering with oxide targets is also not easy to control, as sputter voltages can be high, especially for light doping levels. Arcing events associated with such high voltage cause particulate contamination and can damage targets. In addition, the resistivity of the deposited films, rather than being uniform, tends to map the shape of the magnetron racetrack.
Literature reports of the optimum AZO deposition temperature vary widely: 150 to 250° C. with metal targets (O. Kluth et. al. Thin Solid Films 442 (2003) 80; J. Chang and M. Hon, Thin Solid Films 386 (2001) 79); 250° C. (T. Minami et. al. J. Cryst. Growth 117 (1992)) to 450° C. (Berginski, et. al. Thin Solid Films 516 (2008) 5836) with ceramic targets and RF sputtering, and 150 to 400° C. with ceramic targets and DC sputtering (Minami, supra). Resistivity is reported to increase above the optimum temperature due to formation of scattering centers (J. Chang and M. Hon, Thin Solid Films 386 (2001) 79). The cause of the scattering has been reported as ionized impurity scattering (J. Chang and M. Hon, Thin Solid Films 386 (2001) 79; T. Minami et. al. J. Cryst. Growth 117 (1992) 370) and excessive grain boundary scattering (J. Chang and M. Hon, Thin Solid Films 386 (2001) 79) due to segregation of dopants and diffusion of alkali contaminants from the glass substrate.
The specific attributes required for a TCO coating for silicon tandem deposition are generally low particulate and defect rates to minimize shunting, low enough resistivity to minimize optical absorption, an absorption edge far enough into the UV to maximize light collection by the aSi cell, a plasma frequency low enough into the infrared to maximize light collection by the mc-Si cell, and tunable roughness to control scattering. Typically the sheet resistance of TCO coatings for silicon tandem is in the range of 8-20 Ohm/sq. Module geometry will determine the optimized sheet resistance, as too high sheet resistance increases undesirably the series resistance of the cells, while too low sheet resistance increases optical absorption undesirably. The optimized TCO is thus a tradeoff of these parameters, as increased doping increases free carrier concentration and lowers resistivity until impurity scattering dominates. The increased doping also increases the band gap due to the Burstein-Moss effect, but also increases the plasma frequency, reducing transmission in the IR.
Environmental durability is also required as silicon tandem panels generally must be rated for 20 years or more lifetime. ZnO is very soluble in any acidic solution, so control of morphology and density are key to minimizing dissolution rate.
Light scattering models show it is desirable to have a relatively thick TCO, with roughness of long spatial frequency to scatter light to higher angles. Sputtered AZO films are relatively smooth compared to the roughness required for light scattering. AZO TCO's for silicon-tandem PV are typically deposited at a higher thickness and then wet etched in dilute HCl to achieve the desired roughness. It has been shown that etching smooth long period craters requires a dense columnar film (Berginski, et. al. SPIE 6197 Y1 (2006); O. Kluth et. al. Thin Solid Films 442 (2003) 80). The optimum geometry has been described as lateral sizes of 1-3 μm and mean opening angles of 120-135° (Berginski, et. al. Thin Solid Films 516 (2008) 5836).
It would be desirable to provide improved AZO TCOs and methods of depositing improved AZO TCOs, capable of simultaneously optimizing transparency, conductivity and surface morphology.